4.4 Sample size calculation The sample size calculation for the prospective randomized controlled trial is based on a comparison of two proportions. The proportion of patients developing hypoxemia during sedation is reported between 47% and 51% in observational studies.1 2 Qadeer et al. reported a reduction of hypoxemia (saturation <90%) with additional capnography monitoring from 69% to 46% and Lightdale et al. reported a reduction of hypoxemia (saturation <95%) in children from 24% to 11%.4 5 Based on an observation of daily practice in 58 patients at the CASA clinic Leiden hypoxemia (oxygen saturation < 90%) was recorded in 12 patients, corresponding to an incidence of 20.7%. To show a decrease in hypoxemia from 20% to 10% with a significance level of 0.05 and a power of 0.80, 219 patients are required within each arm, yielding 440 patients to be included in total. Give the annual number of 1600 patients and expecting a participation rate of 60%, we expect to include these patients within a period of 24 weeks.